To minimize pollution of soil and water, an underground storage tank containing hydrocarbon fuels, organics, toxics, or hazardous materials must be tested periodically for leakage, for the life of the tank, in accordance with regulations of the federal Environmental Protection Agency.
Field testers can ascertain leakage in underground storage tanks by a hydrostatic test. The test typically involves topping off the tank with more of the liquid that is in the tank or another acceptable liquid and then applying pressure to the tank for a certain a period of time. If there is no leakage, the pressure should remain unchanged.
However, if the head space in the tank contains vapor, the leakage test result may be false or inaccurate due to condensation or vaporization, for example due to temperature change while the test is being run. It is therefore necessary to remove such vapor before running such underground tank leakage tests. At present, such vapor is purged by first opening or "cracking" the manhole or other tank nozzle to allow the vapor to be displaced as the tank is filled up with a liquid--a time consuming, expensive and sometimes hazardous procedure.